1. Field of the Invention
This invention relates to infrared to visible optical radiation transducers, and more particularly to such transducers that are sufficiently compact and light weight to be incorporated into goggles.
2. Description of the Related Art
An infrared (IR) vision capability is very useful for night vision and other low visibility conditions, such as fog or dust. Most available night vision goggles are large and heavy, and operate with a CRT type display that requires a relatively large amount of power and high voltage. These devices cannot operate under totally dark conditions, such as cloudy nights, since they are based upon light amplification. They are also not very useful under low visibility conditions such as fog, since they are limited to the detection of the visible portion of the spectrum. IR sensitive goggles would overcome these limitations, since they would not require an active illumination source such as star light, and IR radiation is capable of penetrating air turbulence and fog with a significantly reduced scattering.
While several types of IR imagers have been developed, they are not easily incorporated into a head mounted display system. They generally involve a narrow bandgap semiconductor material such as HgCdTe, which must be cooled to prevent it from becoming too conductive at room temperature. Cooling, however, results in a relatively large size and weight requirement and a significant power consumption that is impractical for goggles. Pyroelectric IR detectors and bolometers have also been developed recently that provide an electrical output in response to an IR input. Pyroelectric materials include potassium tantalate niobate, barium strontium titanate, lead zirconate titanate, triglycine sulfate, lead titanate and lithium tantalate, while bolometer arrays are described in U.S. Pat. No. 5,010,251, issued on Apr. 23, 1991 to Grinberg et al. and assigned to Hughes Aircraft Company, the assignee of the present invention. To transfer the IR image from such a sensor array to a separate visible display, a complicated matrix-scanning readout circuit has previously been required. Since the signal generated in uncooled thermal detectors is relatively low, a pixel-amplifier array is also normally needed in the readout scheme. The matrix-scanning technique is described in Scribner, "Infrared Focal Plane Array Technology", Proc. IEEE, Vol. 79, 1991, pages 66-85. Unfortunately, the complicated readout circuitry tends to degrade the yield and to reduce the reliability of such uncooled IR focal plane arrays.
Outside the field of IR detection, protective goggles have been developed to protect the wearer from intense radiation. Such goggles, described in U.S. Pat. No. 5,081,542, issued Jan. 14, 1992 to Efron et al. and also assigned to Hughes Aircraft Company, employ a liquid crystal light valve (LCLV) to provide an image of a scene. The goggles respond to input radiation in the visible to near infrared range to modulate a visible optical readout. Lasers or other high intensity radiation sources are rejected by absorbing their energy in the LCLV's photoconductive layer, thereby protecting the wearer's eyes.
The basic LCLV mechanism is described in Efron et al., "The Silicon Liquid-Crystal Light Valve", Journal of Applied Physics, Vol. 57, No. 4, 1985, pages 1356-1368. It employs a bulk photosensitive layer that converts an optical image into a two-dimensional photogenerated charge pattern, which is transferred through the photosensitive layer to activate a thin liquid crystal (LC) layer. An output image that replicates the input image is generated by modulating an independent light source with the LC layer. Since the input and output images are isolated from each other, various optical functions such as optical amplification, optical limiting, wavelength conversion and incoherent-to-coherent image conversion can be performed, as described in Efron, "Real-Time Signal Processing for Industrial Applications", Proc. of the SPIE, Vol. 960, 1988, pages 180-203. The LCLV has been used as a high intensity, high resolution, large screen projector display, and also for visible-to-IR image conversion (see Efron et al., "Liquid-crystal-based visible-to-infrared dynamic image converter", Optical Engineering, Vol. 24, No. 1, 1985, pages 111-118). An electrically addressed LCLV has also been developed in which a charge coupled device (CCD) array replaces the photoactive array on the input side of the light valve. The CCD-LCLV is described in Welkowsky et al., "Status of the Hughes Charge-Coupled Device-Addressed Liquid Crystal Light Valve", Optical Engineering, Vol. 26, No. 5, 1987, pages 414-417, and in Efron et al, "The Charged-Coupled-Device-Addressed Liquid Crystal Light Valve: An Update", Proc. of the SPIE, Vol. 1455, 1991, pages 237-247. Unfortunately, none of these LCLV devices has an IR detection capability.